Understanding the genetic basis of Ramularia disease resistance in barley

Abstract

Since the 1980’s, barley (Hordeum vulgare) production across many regions in the world, has been facing challenges posed by the fungus Ramularia collo-cygni, which causes Ramularia leaf spot (RLS). The fungus causes substantial yield losses ranging from 20% to 70% and reduces grain quality. The appearance of fungicide resistance in R. collo-cygni populations to a number of fungicide actives, together with the lack of known genetic resistance in widely grown barley varieties, indicates limited options to control this disease in the medium to long-term. This highlights the importance of investigating the potential for genetic control of RLS and gain an improved understanding of the host/pathogen interaction.

In this study, one major quantitative trait loci (QTL) involved in RLS resistance was identified, by using a genome-wide association scan (GWAS) on 238 spring barley varieties. Based on the GWAS results, a subset of the identified spring barley cultivars was tested in field trials in both Scotland and Germany between 2021 to 2022. The results on disease development during both seasons, support the conclusion that resistant QTL carrying barley lines exhibit an increased resistance to RLS. This was supported by results from controlled inoculation experiments in a subset of spring barley seedling experiments. Results at both the adult and seedling stage of barley found that R. collo-cygni-DNA did not correlate with symptom expression, suggesting that endophytic colonisation by the fungus may not always lead to the appearance of symptoms.

The observations from previous studies and the field trial in 2021 of this study, have linked the appearance of RLS symptoms late in the season to monocarpic senescence. Therefore, this study examined the role of senescence during RLS development and fungal colonisation under controlled R.collo-cygni-inoculation studies. This study found that host senescence is prematurely induced by R. collo-cygni in various spring barley genotypes differing in suceptibility to the fungus. Moreover, results have indicated that senescence is induced both in partially resistant and susceptible barley genotypes, suggesting that other factors contribute to the fungus transitioning to the necrotrophic lifestyle. Indeed, delayed leaf senescence experiments in this study showed that RLS symptoms and fungal DNA were increased, whereas during early senescence RLS symptoms were reduced and fungal colonisation was increased. Contrasting the hypothesis that extending the biotrophic phase of R. collo-cygni will lead to increased resistance to RLS and highlighting that other factors contribute to disease development.

Increased resistance to R. collo-cygni in barley plants in controlled inoculation experiments was also found post foliar treatment with the ethylene precursor 1-Aminocyclopropane-1-carboxylic acid (ACC). Furthermore, gene expression analysis of barley ethylene response factors in this study, indicated that one ethylene response factor was upregulated in response to foliar ACC treatment. Altogether, these results suggest a putative ET-mediated disease resistance to RLS and represent potential targets for barley breeding programmes.